'85 300D 123.133

Tachometer has stopped functioning. Somewhere along the way I had a breakdown of how to figure out the problem, as this has happened before. The last time I used compressed air and cleaned the sensor mounted above the ring gear, while it was mounted on the bell housing. This time, compressed air does not work.
I looked in the archives, only found information relating to the tach sensor mounted on the drivers side fender next to the glow plug relay. In a way, the car has this, a three wire harness coming off the balancer, and two separate wires leading into the main harness. There is no circuit board.
I have the first version of the 123 CD, and cannot find a thing related to the tachometer. There are several pages missing from the first versions, Mercedes came out with updated CD's later.
With the tachometer inop, the a/c compressor will not engage. Are there any suggestions? Specs, other ways to test the sensor, tach ???????
Alan